Description
The inflationary diffusion of scalar fields with discrete symmetries can generate a gas of closed domain walls after inflation, which later collapse into primordial black holes (PBHs) upon horizon re-entry. This mechanism predicts a distinctive, nearly flat PBH mass distribution, avoiding some issues of critical collapse. We show that QCD axion models with decay constants near the inflationary Hubble scale (f_a \sim 10^8 GeV) can produce PBHs making up ∼1% of dark matter, consistent with CMB isocurvature bounds.
